Sexual dimorphism in humans is determined by a pair of heteromorphic sex chromosomes, the X and the Y chromosomes. The dioecious plant, Silene latifolia, has a similar sex determining mechanism as humans in which the expression of genes from a heteromorphic, albeit larger, Y chromosome defines males. Unlike the human X/Y chromosome pair, the X and Y chromosomes of S. latifolia are the result of a relatively recent evolutionary event, thus offering a unique window through which to observe initial events leading to the divergence of the X and Y chromosome. We will test the hypothesis that the Y chromosome of S. latifolia, like that in humans, is functionally coherent. That is, the genes required for the initiation and proper development of male sex organs have collected and reside on the Y chromosome, and are arranged in discrete functional units. We propose experiments to identify genes that are expressed specifically during the development of male floral organs. We will then determine the chromosomal linkage of such genes to ascertain whether they are primarily Y-linked or autosomal in origin. Finally, we will map the relative positions of Y-linked sequences on the Y chromosome to determine if they are located in previously described functional regions. If the Y chromosome is functionally coherent, we expect genes involved in male development will have accumulated on the Y and are organized in discrete functional units.